Cheapest? The Nikon I referenced above. They’re about $250-300 used on eBay. That’s what I successfully carried for over 20 years up here on everything (moose, sheep, goat, caribou, bears). Got it as a birthday present when I was 10.
I used to have the big Meopta spotter. Awesome glass but I didn’t pack it all the time. I picked up a used Razor 50mm for $500. It never leaves the pack. It’s so light it would be silly not to carry it. I always have a tripod on me for the binos anyways. It’s perfect for my needs as I’m not real picky on animals.
I hunted for the past 14 years with a B & L Elite 15-45x60 (similar scope to the NIkon SLDMTN referenced). It was used on 3 successful Stone's hunt. In most conditions we had to be under 400 yards to age a ram. The sharpness deteriorated above 40x.
Last year I ordered a 15-45x65 Vanguard. While the picture was brighter and a wider FOV than the B&L it could not pick up as much detail on a pine cone at 200 yards so I sent it back. This winter I tried a Athelon Ares 15-45x65 which is brighter and sharper to my eye, so I kept it (and picked up an extra 1lb of weight).
So to answer the OP - yes I think a scope in the $400 plus range is worth packing.
I had a leupold gold ring 15-30x50 compact and Iiked it. You can pick one up second hand for 3-400 and if you don’t like it you can sell it for that. Can’t go wrong with it if you are just getting into spotters....I upgraded to a Swarovski atx 85 but I used my gold ring for years while I saved up. Sold mine for what I paid for it and used it for a couple years....I think that scope weighs in at around 22oz....it won’t be a very last/first light scope but you can only get so much for that price point and I was never disappointed. Good luck with the choice